Broadcasting services provide for the transmission of media data, for example streaming audio and/or video data, from typically a single data source to multiple receivers. Modern broadcasting services often make use of a wireless technology to transmit the media data over at least an essential segment of the way from the data source to the receivers. Wireless broadcasting services may not only be provided by classical radio stations and television networks, but also by mobile networks, for example GSM (Global System for Mobile Communications) or UMTS (Universal Mobile Telecommunications System) networks.
Each broadcasting service is provided into a broadcast area, i.e. into a geographical area in which the media data can be received. In case of a PLMN (Public Land Mobile Network), the broadcast area may comprise the whole network. On the other hand, a broadcast area may be configured to be as small as a single radio cell of a cellular network. In general, a broadcast service area comprises a reasonable part of a PLMN.
Each network cell is served by a transmitter site comprising at least one transmitter or transmitter station. For example, a transmitter site in a GSM network comprises a BTS (Base Transceiver Station) which may be controlled by a BSC (Base Station Controller); a transmitter site in a UMTS network comprises a Node-B which may be controlled by an RNC (Radio Network Controller). Consequently, multiple transmitter sites are required at least for broadcast areas comprising many cells. The multiple transmitter sites may use the same frequency resource, i.e. operate on the same frequency; this operational mode is called Single Frequency Network (SFN).
To minimize interference, the transmitter sites of an SFN are typically run synchronised with each other, i.e. all transmitters of the multiple sites synchronously transmit the same broadcast signal. This may be achieved for example by using GPS (Global Positioning System) or a reference clock provided by one of the transmitter sites or by a control node in the broadcast network. A receiver, e.g. a receiver component in a user equipment of the mobile network, thus receives broadcast signals of multiple nearby transmitter sites.
The receiver may superpose the signals received from transmitter sites within a certain distance from the receiver. Signals received from more distant transmitter sites contribute to the interference level at the location of the receiver. As an example, an OFDM (Orthogonal Frequency Division Multiplex) broadcast system uses a particular guard interval the length of which determines the maximum distance for constructive superposition. Examples of OFDM-based broadcast systems are DVB-T (Digital Video Broadcasting-Terrestrial) and DAB (Digital Audio Broadcasting).
To achieve at the same time a predetermined minimum reception quality in terms of a maximum bit or packet error rate over the entire broadcast area and a high spectral efficiency (ratio of media data bit rate to required transmission bandwidth), an important measure is provided by the Signal to Interference plus Noise Ratio (SINR). The lower the SINR, the higher the error rate. In order to achieve a desired error rate and bit rate for a low SINR, a transmission mode is required which utilizes more radio resources than a transmission mode adequate for a high SINR, thus decreasing the spectral efficiency.
The transmission mode determines the coverage of the transmission, i.e. the zone around a transmission site wherein reception of the transmitted broadcast signal is possible for a receiver with a bit or packet error rate below a predetermined quality threshold. In cellular networks, radio resources may for example be specified according to one or more of the aspects time, frequency, transmit power and spreading code. Accordingly, the transmission mode is specified by choosing for example a particular transmit power, a particular channel coding (e.g., higher or lower code rates), particular spreading codes with specific spreading factors, etc.
By its definition, the SINR at a receiver at a particular location in the broadcast area increases with the received aggregate useful signal, which is the superposition of all useful signals from individual transmitters, and decreases with the aggregate interference, which is the superposition of all interfering signals from individual transmitters, and with the noise level, which may be assumed to be a location independent constant. Depending on the deployment of the broadcast network, either the interference can dominate the noise or vice versa. If the noise dominates, the SINR increases with decreasing distance to the transmitters. Even if the interference dominates, generally the SINR increases with decreasing distance to the transmitters, because the interference will increase less than the useful signal.
Accordingly, the minimum SINR in an SFN broadcast area is generally dependent on the inter-site distance (ISD), defined as the average distance between any pair of transmitter sites in a region of the broadcast area. The SINR typically decreases with increasing ISD. The minimum SINR in a broadcast area typically occurs in a region of the broadcast area with a large ISD.
The minimum SINR (which may be a percentile) in a given broadcast area is generally determined by measurement. Then a transmission mode is chosen to achieve the desired error rate. However, in an SFN network, increasing for example the transmit power might be sufficient to increase the Signal to Noise Ratio, but at the same time may increase interference and thus lead to a decreasing SINR. The minimum SINR in the broadcast area may also decrease (and may occur at different locations in the broadcast area).
Using different transmission modes on the same radio resources would make it more difficult to superpose the signals received from transmitter sites using the different modes, in particular in the case of SFNs. In SFN broadcast areas comprising a region or regions of small ISD (e.g., cities) and a region or regions of large ISD (e.g., rural areas), it is thus difficult to find an optimal transmission mode. In case a transmission mode is chosen which is sufficiently robust to achieve full coverage also in the region of large ISD, this transmission mode will not exploit the higher SINR achieved in regions of small ISD and therefore the spectral efficiency in these regions will be lower than in the case that a less robust transmission mode had been used that still achieves full coverage in regions of small ISD.
In regions of small ISD the SINR is high. This allows to achieve full coverage even for transmission modes which are less robust and can therefore provide a higher throughput. These transmission modes are however not sufficiently robust for the regions of large ISD and therefore cannot achieve full coverage in these regions.
Accordingly, there is a need for a technique for controlling a wireless broadcast transmission of media data via multiple transmitter sites into a broadcast area with different inter-site distances, wherein the technique provides for a high spectral efficiency in regions of small ISD and at the same time for an acceptable reception quality in regions of large ISD.